1. Field of Invention
The invention relates to a power conversion apparatus and a control method thereof and, in particular, to a power conversion apparatus and a control method thereof which are applied to an aerogenerator.
2. Related Art
In recent years, owing to the rising awareness of environment protection and the depletion of fossil energy (e.g. oil, coal), the whole world is conscious of the importance of the alternative energy development. Since wind energy is inexhaustible in supply and always available for use, there is no doubt about the depletion of the energy source and no issue of energy monopoly. Therefore, the countries of the world energetically develop the aerogeneration system, and expects that the reliance on the fossil energy will be reduced by the increasing use of the wind energy. The power generation principle of an aerogenerator is described as below. The wind pressure caused by the flowing air is used to rotate the blades of the wind turbine so as to generate the mechanical energy, and then the mechanical energy is used to rotate the rotor of the power generator so as to generate the electric energy. Generally, before this electric energy outputted by the aerogenerator is used for the storage, loading or electric network, it needs to be converted by a power conversion apparatus.
FIG. 1A is a characteristic curve diagram of the output power PWG (kW) and the output voltage VWG—rms (root-mean-square value) of a conventional aerogenerator G under different wind speeds, and FIG. 1B is a characteristic curve diagram of the output voltage VWG—rms (root-mean-square value) and the output current IWG—rms (root-mean-square value) of a conventional aerogenerator G under different wind speeds.
As shown in FIGS. 1A and 1B, under different wind speeds, the aerogenerator G has different voltage and current values at the maximum power point. The maximum power point corresponding to each of the wind speeds can be connected together by an approximation line, i.e. a maximum power approximation line. Besides, the voltage value corresponding to the maximum power point under every wind speed can be found by the approximation line, and then the current value corresponding to the maximum power point can be obtained by using the voltage value and referring to the voltage/current characteristic curve in FIG. 1B.
Moreover, FIG. 1C is a schematic diagram of a conventional power conversion apparatus 1. Herein, the power conversion apparatus 1 can receive the electric energy outputted by an aerogenerator G and then supply it to a loading L after the power conversion.
In the conventional art, in order to achieve the functions of maximum power tracking and three-phase power factor correction (PFC) to enhance the electromechanical conversion efficiency of the aerogenerator G, two converters are usually required for the implementation of the power conversion apparatus. One is a three-phase PFC converter 11 and the other is a maximum power tracking converter 12. The three-phase PFC converter 11 is used to make the three-phase current phase and the voltage phase outputted by the aerogenerator G close to each other or the same, so as to achieve a higher power factor to enhance the efficiency. The maximum power tracking converter 12 is used to make the aerogenerator G capable of operating at the maximum power point under different wind speeds, and that means the aerogenerator G operates at the maximum power approximation lines of FIGS. 1A and 1B. Therefore, the electromechanical conversion efficiency can be enhanced by the PFC converter 11 and the maximum power tracking converter 12 of the power conversion apparatus 1.